A transducer may be broadly defined as a device which converts one form of input energy to a different form of output energy. An electromechanical transducer, when configured to convert electrical energy to mechanical energy, may operate on the principle of electrostatic attraction caused by two opposing and oppositely charged conducting plates. For example, as electrical energy is input to the transducer in the form of a voltage applied between the plates, the plates are drawn together If the plates are free to move together, the input electrical energy is converted into mechanical energy
The plates of an electromechanical transducer may also be used to generate electrical energy from an input of mechanical energy. For example, the plates are first charged by an electrical voltage applied to the plates. The plates may then be disconnected from the charging source and mechanical energy used to further separate the plates. As the plates are separated, the voltage between the plates increases thereby converting the mechanical energy to electrical energy.
Accordingly, an electromechanical transducer may be used either as an actuator or a sensor. As an actuator, the transducer may convert electrical power into mechanical motion, and, as a sensor, the transducer may convert mechanical motion into an electrical signal.
Electromechanical transducers have been developed which convert electrical energy to mechanical motion and ultimately to acoustic energy by the application of a voltage between a pair of spaced parallel conducting plates. If the plates are flexible or otherwise configured to allow motion, the plates are drawn together by the force of electrostatic attraction when the signal voltage is applied between the plates. See, for example, U.S. Pat. No. 3,008,014 to Williamson et al., which discloses an electrostatic transducer used in entertainment loudspeaker systems to convert electrical signals into sound. Since the driving voltage required to move the plates is related to the square of the separation between the plates, transducers of the type described in Williamson et al. require large and potentially hazardous driving voltages.
As is known in the art, the force generated by a pair of opposing parallel plates is inversely proportional to the square of the distance between the plates. The force generated by the plates increases by a power of two for a corresponding linear decrease in the separation between the plates. Accordingly, very large forces can be developed as the spacing between plates is decreased. In addition, for a given force, as the separation is decreased, the driving voltage can be reduced.
To obtain useful forces and physical displacements as the size of the separation between plates is reduced, a large number of plates must be concatenated or stacked together U.S. Pat. No. 2,975,307 to Schroeder et al. discloses an electrostatic transducer having a large number of stacked plates, each plate with an individual and discrete external wiring connection to the source of the driving voltage, and each pair of adjacent plates having a series of individual and discrete separators placed in a precise pattern therebetween.
Unfortunately, it is difficult to connect each of the plates to a supply voltage in a array having a large number of closely spaced plates. In an array of closely spaced stacked plates, many hundreds or even thousands of discrete connections must be made to each plate in the stacked array of plates. In addition, the physical assembly of such a large number of plates, spacers, and other components of such small dimensions is extremely difficult and not, therefore, amenable to efficient manufacturing. An array of closely spaced stacked plates is also typically limited to bistable operation, that is, the array is either fully contracted or fully relaxed.
Conventional electrical, hydraulic and pneumatic actuators are adequate for a wide assortment of applications. An electric motor, and more particularly the electric stepper motor, is frequently used where a predetermined amount of rotation is required. The electric motor can also be interfaced to gears, levers, or other structures to obtain a desired movement of an object. A solenoid typically includes a coil and a spring-loaded plunger or piston which is drawn into the coil by a magnetic field generated by an electric current flowing through the coil windings. Thus, the motor and solenoid consume power whenever their windings are energized. Both the electric motor and solenoid are subject to sliding friction Therefore, the performance of such actuators for fine positioning--micropositioning--is unacceptable. The use of such actuators to obtain precision movement on the order of several microns over similar small distances is impractical. In addition, the bulk and weight of conventional actuators often far exceeds the size of the object to be moved. Inefficiencies thereby result in raw materials for constructing the actuators and in the energy to operate them.
In addition to simple micropositioning, the art has developed two-dimensional positioners for use in, for example, microscopic scanning. Such scanning requires that small displacements are made without binding or overshoot as may typically be caused by sliding friction. Scanning positioning has been done with electric motors, electromagnetic drives, and piezoelectric drives. These positioners are often difficult and expensive to manufacture and maintain and may still be subject to binding or overshoot, such as caused by sliding friction.
The microelectronics art has attempted to overcome some of the limitations of conventional actuators for micropositioning and other actuation applications. For example, the Massachusetts Institute of Technology, has fabricated an eight-pronged rotor that spins around a center bearing as more fully described in Howe, Muller, Gabriel, and Trimmer, "Silicon Micromechanics: Sensors and Actuators on a Chip," IEEE Spectrum, pp. 29-31 and 35 (July, 1990). As described, friction and wear at the bearing points of rotating or sliding structures at these dimensions are of great importance and may readily cause the failure of such a device after a few minutes of operation. However, to the best of applicant's knowledge, microelectronic techniques have not been the basis for the design and fabrication of electromechanical transducers comprising an array of a large number of closely spaced parallel plates in a deformable plastic and metal structure.